1. Field of the Invention
The present invention relates to an exhaust gas recirculation system for an internal combustion engine. The exhaust gas recirculation system is referred to as the EGR system in this specification.
2. Description of Related Art
An EGR system for recirculating part of exhaust gas of an internal combustion engine to an intake system has been known hitherto. Two types of the EGR systems have been used. One is a high pressure EGR system in which a high pressure exhaust gas exhausted from a combustion chamber is directly supplied to a downstream portion of a throttle. The other one is a low pressure EGR system in which exhaust gas (a relatively low pressure gas) passed through an exhaust gas cleaner is supplied to an upstream portion of a supercharger (refer to JP-A-5-187329).
In the low pressure EGR system, exhaust gas passed through a turbine of a supercharger and an air cleaner having catalysts is supplied to the upstream portion of the supercharger compressor in an intake pipe. If foreign particles are mixed with the exhaust gas in the turbine portion or the air cleaner portion in an exhaust pipe, the foreign particles enter into the compressor portion of the supercharger in the intake pipe. The foreign particles may cause a malfunction in the supercharger or in the engine.
In the low pressure EGR system shown in JP-A-5-187329, a filter having catalysts is disposed in a passage connecting an exhaust system and an intake system. Although carbon particles contained in the exhaust gas are burnt in the filter, relatively large particles contained in the exhaust gas cannot be removed in the filter. These particles may clog small passages in the filter. In addition, since the Filter contains catalysts, the filter is expensive.